Method for treating surface of metal product

ABSTRACT

Disclosed herein is a method for treating the surface of a metal product, the method comprising the steps of: degreasing the surface of the metal product, and removing a material used to degrease the surface, followed by neutralization and washing; spraying either a mixture of ethanol (C 2 H 5 OH), acetone (CH 3 COCH 3 ) and water or alcohol onto the surface of the washed metal product; and forming an oxide layer on the surface of the metal product by firing or heat-treating the metal product resulting from the step of spraying.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for treating the surface of a metal product, and more particularly to a method for treating the surface of a metal product, in which a oxide layer and the like can be rapidly formed on the surface of the metal product.

2. Description of the Prior Art

About ¾ of chemical elements known to date are metal elements, including aluminum (Al), iron (Fe), copper (Cu) and the like. These metal elements generally exist in a crystalline solid form and have a relatively simple crystalline structure, and thus the arrangement of atoms thereof is dense and highly symmetrical. Also, the number of valance electrons of each metal atom is ½ or less of the maximum number thereof, and thus metal elements do not easily form a compound with each other. However, metal elements easily form bonds with oxygen and sulfur, which are non-metal elements having 2/1 or more of the maximum number of valence electrons.

Metal materials are mainly used for automobile and aircraft parts, mobile phone cases, notebook computer cases, spectacles frames, kitchen utensils and the like. Specifically, magnesium alloys, aluminum and aluminum alloys, titanium and titanium alloys, iron and iron alloys, and copper and copper alloys are widely used in various applications.

In a conventional method, an oxide layer is formed on the surface of a metal by a surface treatment method such as anodic oxidation, anodizing or plasma electrolytic oxidation.

For example, a product made of a magnesium alloy is surface-treated by a surface treatment method such as anodizing or plasma electrolytic oxidation to form a thin layer such as a MgO layer.

In other word, an electrolyte that is used in the surface treatment method is mainly sodium hydroxide (NaOH). In a surface treatment process employing sodium hydroxide as an electrolyte, a hydroxyl group (OH—) separated from the sodium hydroxide is bonded to the surface layer of a magnesium alloy product by a strong electric field to form an oxide layer of MgO and Mg(OH)₂on the surface layer.

However, the above-described conventional method had a problem in that the process of forming an oxide layer on the surface of a product is time-consuming, and thus the surface of the product cannot be quickly treated.

Another conventional surface treatment method comprises applying a coating to the surface of a metal product to form a coating layer or forming a metal coating layer on the surface by a sputtering process. However, the method of forming the coating layer or the meal coating layer on the surface of the metal surface has a shortcoming in that the adhesion between the metal surface and the coating layer or the metal coating layer is low so that the coating layer and the metal coating layer are easily peeled off.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made in view of the problems occurring in the prior art, and it is an object of the present invention to provide a method for treating the surface of a metal product, in which an oxide layer can be rapidly formed on the surface by spraying a surface treatment solution onto the surface and subjecting the sprayed solution to direct firing or heat treatment.

Another object of the present invention is to provide a method for treating the surface of a metal product, in which a coating layer, a metal coating layer or an electrodeposited layer can be securely formed on the oxide layer formed as described above.

To achieve the above objects, the present invention provides a method for treating the surface of a metal product, the method comprising the steps of: degreasing the surface of the metal product, and removing a material used to degrease the surface, followed by neutralization and washing; spraying either a mixture of ethanol (C₂H₅OH), acetone (CH₃COCH₃) and water or alcohol onto the surface of the washed metal product; and forming an oxide layer on the surface of the metal product by firing or heat-treating the metal product resulting from the step of spraying.

The surface treatment solution may be a mixture of ethanol (C₂H₅OH), acetone (CH₃COCH₃) and water, wherein the amount of the ethanol is 1-50 times the volume of the water and the amount of the acetone is 0.05-1 time the volume of the ethanol.

The step of forming the oxide layer may be carried out by directly firing the metal product resulting from the spraying step in a direct firing furnace at 200˜260° C. for 10-15 seconds. Alternatively, the step of forming the oxide layer may be carried out by heat-treating the metal product resulting from the spraying step in a heat treatment furnace at 150˜200° C. for 5-20 minutes.

In addition, the method for treating the surface of the metal product according to the present invention may further comprise washing and drying the metal product resulting from the oxide layer forming step, and then forming a coating layer, an electrodeposited layer or a metal coating layer on the oxide layer on the surface of the metal product by a coating process, an electrochromic process or a metal coating process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged photograph showing the surface of a magnesium alloy product treated by the surface treatment method of the present invention.

FIG. 2 is an enlarged photograph showing the section of a magnesium alloy product treated by the surface treatment method of the present invention.

FIG. 3 is an enlarged photograph of the section of a magnesium alloy product having a coating layer formed thereon, treated by the surface treatment method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiment of a method for treating the surface of a metal product according to the present invention will described in further detail.

In the method for treating the surface of a metal product according to the present invention, a surface treatment solution or alcohol is sprayed onto the surface of the metal product to remove oil or stains from the metal product surface, before the surface is treated in a direct firing furnace or heat treatment furnace having an internal space which is maintained at a specific temperature.

Specifically, the present invention comprises degreasing the metal product surface, and removing a material used to degrease the surface, followed by neutralization and washing, like a conventional method. For example, the metal product is pretreated by the steps of removing oil or stains from the product surface using trichloroethylene, removing the trichloroethylene, neutralizing acetone, and washing the product surface.

In the step of removing oil from the product surface, the product is treated with a trichloroethylene (TCE; (C₂HCl₃) solution.

For reference, trichloroethylene (C₂HCl₃; TCE) is a colorless transparent liquid having a sweet smell. It is an organic solvent which is used for degreasing and washing of metal mechanical parts, drying of metal surfaces, washing and dyeing in the textile industry, dissolution, dilution of lacquer, washing of glass or optical devices, removal of fats from leather, etc.

Such trichloroethylene is used to remove decrease the surfaces of automobile and aircraft parts, mobile phone cases, notebook computer cases, spectacles frames, kitchen utensils and the like.

Then, the step of removing trichloroethylene from the degreased surface is carried out. In this step, the degreased metal product is treated with acetone to remove the trichloroethylene solution from the surface.

Acetone used to remove the trichloroethylene solution needs to be neutralized, before the product is washed with water. In the step of neutralizing acetone, the product surface is treated with a methanol solution to neutralize acetone used to remove trichloroethylene.

For reference, methanol (CH₃OH) is an alcohol having the simplest structure among alcohols and is also called methyl alcohol. It is also called wood spirit, because it is obtained from pyroligneous liquid produced by wood distillation. It is contained in various natural materials and exists in the form of either methyl esters of various carboxylic acids such as salicylic acid or various methyl esters.

Methanol is a colorless transparent volatile liquid having a peculiar smell. It is easily miscible with solvents such. as water, ethanol, benzene or ether and may be added to gasoline to provide a cold-resistant. fuel for automobiles. It may also be added to ethanol to provide modified alcohol.

After acetone on the product surface has been neutralized. with methanol, The product needs to be washed. In this washing step, the product resulting from The neutralization step is washed with water, followed by removal of the water.

In the present invention, a step of spraying a surface treatment solution or alcohol onto the surface of the product is carried out in order to facilitate the treatment of the product surface, before the product washed in the washing step is placed in a direct firing furnace or a heat treatment furnace.

The surface treatment solution. that is used in the solution spray step is a simple mixture of water, ethanol. (C₂H₅OH) and acetone (CH₂COCH₂), wherein the amount of the ethanol is 1-50 times the volume of the water and the amount of the acetone is 0.05-1 time the volume of the ethanol.

In addition to the surface treatment solution consisting of the mixture of ethanol, acetone and water, alcohol may also be sprayed onto the metal product surface in the solution spray step. Even when alcohol is sprayed onto the metal product surface, an oxide layer can be easily formed on the surface as described below.

Ethanol that is used in the solution spray step influences the stability of flames and the removal of stains and serves to reduce the surface tension of water. If ethanol is used in an amount smaller than 1 time the volume of water, an oxide layer will not be easily formed on the metal product surface because the amount of hydroxyl (OH—) groups will be insufficient, and the sprayed surface treatment solution will not be uniformly distributed on the product surface due to the surface tension of water, resulting in formation of stains.

If ethanol is used in an amount larger than 50 times the volume of water, an oxide layer will be easily formed due to water, the amount of hydroxyl (OH—) groups will be excessively large, and the stability of boiling point of water and ethanol will be reduced, making it difficult to form a uniform oxide layer.

For reference, ethanol (C₂H₅OH) or ethyl alcohol is a colorless inflammable alcohol compound. It exists as a colorless liquid at room temperature and generates a transparent, light blue flame upon firing. It has a peculiar smell and taste, forms a hydrogen bond, has a melting point of −114.5° C. and a boiling point of 78.32° C., and can dissolve in water, other alcohols, ether and chloroform. In addition, because it generates explosive gas, it can be used as fuel in some internal combustion engines. Further, it is frequently used as a solvent, a disinfectant, fuel or the like.

Acetone that is used in the solution spray step serves to ensure uniform heating temperature (uniform heating power of ethanol) and shorten the surface treatment time. If acetone is used in an amount smaller than 0.05 times the volume of ethanol, the uniform heating power of ethanol cannot be ensured so that the uniform thickness of the oxide layer cannot be ensured, and if acetone is used in an amount larger than 1 time the volume of ethanol, the surface treatment time will be increased due to an excessively large amount of acetone, and bubbles will occur to reduce the uniformity of the surface.

For reference, acetone (CH₃COCH₃) is miscible with most solvents such as water, alcohol or ether and has high volatility at room temperature and high flammability. It is reduced into isopropyl alcohol by most reducing agents, produces tetramethylethylene glycol with sodium amalgam, reacts with sodium bichromate to produce acetic acid and carbon dioxide, and reacts with ammonia to produce acetone amine such as diacetone amine.

In addition, acetone is widely used as a convent in the preparation of plastic or cellulose coatings and is used to dissolve and store acetylene. It is also used as a raw material for organic synthesis. A typical compound produced from acetone is diacetone alcohol which is used as a solvent, a thinner. In addition, acetone easily dissolves in water and organic solvents, and thus is used to wash substances such as paints, which are difficult to wash with water.

After the solution spray step, the step of forming an oxide layer on the product surface. In this step, the metal product is fired or heat-treated to form an oxide layer on the surface of the metal product.

For example, the step of forming the oxide layer is carried out by firing the metal product in a direct firing system having an internal space at 200-260° C. for 10-15 seconds. If the treatment temperature is lower than 200° C., the surface treatment time will be increased, and an oxide layer will not be easily be formed on the product surface, and if the treatment temperature is higher than 260° C., the surface of the product will not be uniform.

The metal product in the direct firing system is directly fired. If the firing time is shorter than 10 seconds, the oxide layer will not be easily formed, and if the firing time is longer than 154 seconds, the surface quality of the metal product will be deteriorated.

In another embodiment of the present invention, the surface treatment solution or alcohol on the surface of the metal product may be heat-treated by a heat treatment process in place of the direct firing method as described above. In this heat treatment method, the metal product treated in the solution spray step is heat-treated in a heat treatment furnace having an internal space at 150-200° C. for 5-20 minutes. if the heat treatment temperature is lower than 150° C., the heat-treatment state cannot be maintained to increase the surface treatment time, and the oxide layer will not be easily formed on the surface of the metal product, and heat treatment at a temperature higher than 200° C. will be insufficient compared to the direct firing method.

Although the heat treatment method as described above requires a longer treatment time than the direct firing method, it can easily remove the surface treatment solution or alcohol from the product surface compared to the direct firing method. The metal product in the heat treatment furnace is indirectly heated. If the heat treatment time is shorter than 5 minutes, the oxide layer will not be easily formed, and the metal product does not need to be heat-treated for longer than 20 minutes, because the surface treatment solution can be completely removed within 20 minutes.

The metal product that is treated in the present invention is made of a material selected from among magnesium alloys, aluminum and aluminum alloys, titanium and titanium alloys, iron and iron alloys, copper and copper alloys, and the like. If the metal product is made of a magnesium alloy, an oxide layer of MgO and Mg(OH)₂ will be formed, and if it is made of aluminum or an aluminum alloy, an oxide layer of Al₂O₃ will be formed. If the metal product is made of titanium or a titanium alloy, an oxide layer of TiO₂ will be formed, and if it is made of iron or an iron alloy, an oxide layer of Fe₃O₄ will be formed. If the metal product is made of copper or a copper alloy, an oxide layer of Cu₂O will be formed.

For example, the surface of a magnesium alloy product is decreased, and then the material used to decrease the surface is removed, followed by neutralization and washing.

Next, the surface of the magnesium alloy product is sprayed with the surface treatment solution and fired by the direct firing method. As a result, as shown in FIGS. 1 and 2, a thin oxide layer having a fine pattern can be rapidly formed on the surface of the magnesium alloy product.

Although FIGS. 1 and 2 show the oxide layer formed on the surface of the magnesium product, an oxide layer may also be formed on the surface of metal products made of aluminum, aluminum alloys, titanium, titanium alloys, iron, iron alloys, copper or copper alloys by the surface treatment method of the present invention.

Thus, due to the oxide layer having a fine pattern, an electrodeposited layer, a coating layer or a painting layer can be securely formed as described below.

After the oxide layer has been formed on the product surface as described above, a washing and drying step and an electrodepositing step are carried out to form an electrodeposited layer on the oxide layer. In the washing and drying step, the metal product treated in the step of forming the oxide layer is washed and then dried, and then an electrodeposited layer is formed on the oxide layer by an electrochromic process.

The washing and drying step is preferably carried out using ethanol, and after washing, the ethanol is sufficiently dried.

In the electrodepositing step, an electrodeposited layer is formed by a conventional method. As shown in FIG. 3, the electrodeposited layer is formed on the oxide layer formed on the surface of the metal product resulting from the washing/drying step. Because the electrochromic process is well known in the art, the detailed description thereof is omitted herein.

In the present invention, the thickness of the electrodeposited layer formed in the electrodepositing step is preferably 10-24 μm. This thickness is the optimum thickness determined in view of surface gloss, durability, and treatment cost. Because the electrodeposited layer is uniformly formed on the oxide layer formed on the surface of the metal product, gloss can be imparted to the surface of the metal product.

A sample obtained by forming an oxide layer and an electrodeposited layer on the surface of a metal product was subjected to a salt spray test (hair-line surface treatment) at 24-hr intervals for 500 hours. As a result, it could be seen that the surface gloss of the electrodeposited layer formed on the surface of the metal product was maintained in a good state.

In another embodiment of the present invention, an oxide layer is formed on the surface of the metal product resulting from the solution spray step, and then a washing and drying step and a coating step are sequentially carried out to form a coating layer on the oxide layer.

After the oxide layer has been formed on the surface of the metal product, the metal product is washed and then dried. After the washing and drying step, a coating process is carried out to form a coating layer on the oxide layer formed on the surface of the metal product.

Examples of the coating process include bake coating, Teflon coating, ceramic coating, power coating, and the like. The step of forming the coating layer on the oxide layer by the coating process is carried out using a known method such as power coating, and thus the detailed description thereof is omitted herein.

In the coating step, the coating layer is formed on the oxide layer using a coating material such as a bake coating, an epoxy coating, an enamel coating or a power coating.

Particularly, the coating layer is preferably formed to the optimum thickness determined in view of surface gloss, durability, and treatment cost. Thus, the coating process needs to be subdivided in view of the gloss and durability of the coating layer.

Specifically, the metal product resulting from the washing/drying step is placed on a conveyer, and the surface of the product is pretreated by an ion gun or nozzle. Then, the metal product is primer-coated to increase the corrosion resistance, after which it is dried indirectly and top-coated to increase the durability of the coating. Then, it is dried indirectly and then glossed, and the coating layer formed on the surface of the product is completely dried.

According to the present invention, the coating layer strongly adheres to the oxide layer having a fine pattern, formed on the surface of the metal product, so that the durability of the coating layer is increased. Thus, the durability of the coating layer can be increased due to the fine pattern of the coating layer.

In another embodiment of the present invention, a metal coating layer is formed on the surface of the metal product by a dry coating process after the washing/drying step.

Examples of the dry coating process include electron beam deposition, ion plating, sputtering, PECVD (plasma-enhanced chemical vapor deposition) and the like. The step of forming a metal coating layer on the oxide layer by the dry coating process is carried out using a known dry coating process such as sputtering, and in this step, a metal coating layer of any one selected from among aluminum (Al), tin (Sn), titanium (Ti), copper (Cu), gold (Au), silver (Ag), nickel (Ni), stainless steel (SUS), and silicon (Si). The dry coating layer such as sputtering is well known in the art, and thus the detailed description thereof is omitted herein.

In the dry coating step, two or more selected from among aluminum (Al), tin (Sn), titanium (Ti), copper (Cu), gold (Au), silver (Ag), nickel (Ni), stainless steel (SUS), and silicon (Si) may be sequentially coated on the oxide layer by the dry coating process to form a plurality of metal coating layers, or various patterns can be formed on the oxide layer using different metal materials The metal coating layer is preferably formed to the optimum thickness determined in view of surface gloss, durability, and treatment cost.

As described above, according to the present invention, an oxide layer having a fine pattern can be rapidly formed on the surface of a metal product, and the durability of a coating layer formed on the oxide layer can be increased due to the fine pattern of the oxide layer. Also, the coating layer of the metal product can show a metal texture and a good appearance.

As described above, in the method for treating the surface of a metal product, foreign matter such as oil is removed from the surface of the metal product, and then either a mixture of water, ethanol (C₂H₅OH) and acetone (CH₃COCH₃) or alcohol is sprayed onto the surface, after which an oxide layer is formed on the surface by the direct firing method or the heat treatment method. Accordingly, the time of surface treatment by direct firing or heat treatment can be significantly shortened, and thus the oxide layer can be rapidly formed on the surface of the metal product.

In addition, on the oxide layer formed on the surface of the metal product, an electrodeposited layer is formed by the electrochromic process to achieve colored gloss. Also, the metal product was subjected to a salt spray test (hair-line surface treatment), and as a result, it could be seen that the surface gloss of the electrodeposited layer formed on the surface of the metal product was maintained for a long period of time.

Moreover, because the oxide layer formed on the surface of the metal product has a fine pattern, the durability of the coating layer or metal coating layer formed on the oxide layer can be easily increased. 

What is claimed is:
 1. A method for treating the surface of a metal product, the method comprising the steps of: degreasing the surface of the metal product, and removing a material used to degrease the surface, followed by neutralization and washing; spraying either a mixture of ethanol (C₂H₅OH), acetone (CH₃COCH₃) and water or alcohol onto the surface of the washed metal product; and forming an oxide layer on the surface of the metal product by firing or heat-treating the metal product resulting from the step of spraying.
 2. The method of claim 1, wherein the surface treatment solution in the spraying step is a mixture of ethanol (C₂H₅OH), acetone (CH₃COCH₃) and water, wherein the amount of the ethanol is 1-50 times the volume of the water and the amount of the acetone is 0.05-1 time the volume of the ethanol.
 3. The method of claim 1, wherein the step of forming the oxide layer is carried out by directly firing the metal product resulting from the spraying step in a direct firing furnace at 200-260° C. for 10-15 seconds.
 4. The method of claim 1, wherein the step of forming the oxide layer is carried out by heat-treating the metal product resulting from the spraying step in heat treatment furnace at 150-200° C. for 5-20 minutes.
 5. The method of claim 1, wherein the method further comprises the steps of: washing treating the metal product resulting from the oxide layer forming step with ethanol, followed by drying; and forming an electrodeposited layer on the oxide layer on the surface of the metal product resulting from the washing/drying step by an electrochromic process.
 6. The method of claim 1, wherein the method further comprises the steps of: washing treating the metal product resulting from the oxide layer forming step with ethanol, followed by drying; and forming a coating layer on the oxide layer on the surface of the metal product resulting from the washing/drying step by a coating process.
 7. The method of claim 1, wherein the method further comprises the steps of: washing treating the metal product resulting from the oxide layer forming step with ethanol, followed by drying; and forming a metal coating layer on the oxide layer on the surface of the metal product resulting from the washing/drying step by a dry coating process. 